The present invention relates generally to a thermal energy transfer device for use within an x-ray generating device or x-ray system and, more specifically, to a heat receptor for use within an x-ray tube or x-ray system.
Typically, an x-ray generating device, referred to as an x-ray tube, includes opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is commonly fabricated from glass or metal, such as stainless steel, copper, or a copper alloy. The electrodes include a cathode assembly positioned at some distance from the target track of a rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode assembly may be stationary. The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or a tungsten alloy. Further, to accelerate electrons used to generate x-rays, a voltage difference of about 60 kV to about 140 kV is commonly maintained between the cathode and anode assemblies. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode assembly at high velocity. A small fraction of the kinetic energy of the electrons is converted to high-energy electromagnetic radiation, or x-rays, while the balance is contained in back-scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from a focal spot, and may be directed out of the vacuum vessel along a focal alignment path. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the vacuum vessel to allow an x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed along the focal alignment path to penetrate an object, such as a hum;an anatomical part for medical examination and diagnostic purposes. The x-rays transmitted through the object are intercepted by a detector or film, and an image of the internal anatomy of the object is formed. Likewise, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at an airport.
Since the production of x-rays in a medical diagnostic x-ray tube is by its very nature an inefficient process, the components in the x-ray tube operate at elevated temperatures. For example, the temperature of the anode""s focal spot may run as high as about 2,700 degrees C., while the temperature in other parts of the anode may run as high as about 1,800 degrees C. The thermal energy generated during tube operation is typically transferred from the anode, and other components, to the vacuum vessel. The vacuum vessel, in turn, is generally enclosed in a casing filled with a circulating cooling fluid, such as dielectric oil or air, that removes the thermal energy from the x-ray tube. The casing also supports and protects the x-ray tube and provides a structure for mounting the tube. Additionally, the casing is commonly lined with lead to shield stray radiation.
As discussed above, the primary electron beam generated by the cathode of an x-ray tube deposits a large heat load in the anode target. In fact, the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, the balance being converted to thermal energy. This thermal energy from the hot target is conducted and radiated to other components within the vacuum vessel. The fluid circulating around the exterior of the vacuum vessel transfers some of this thermal energy out of the system. However, the high temperatures caused by this thermal energy subject the x-ray tube components to high thermal stresses that are problematic in the operation and reliability of the x-ray tube. This is true for a number of reasons. First, the exposure of components in the x-ray tube to cyclic high temperatures may decrease the life and reliability of the components. In particular, the anode assembly typically includes a shaft that is rotatably supported by a bearing assembly. The bearing assembly is very sensitive to high heat loads. Overheating of the bearing assembly may lead to increased friction, increased noise, and to the ultimate failure of the bearing assembly. Due to the high temperatures present, the balls of the bearing assembly are typically coated with a solid lubricant. A preferred lubricant is lead, however, lead has a low melting point and is typically not used in a bearing assembly exposed to operating temperatures above about 330 degrees C. Because of this temperature limit, an x-ray tube with a bearing assembly including a lead lubricant is limited to shorter, less powerful x-ray exposures. Above about 450 degrees C., silver is generally the lubricant of choice, allowing for longer, more powerful x-ray exposures. Silver, however, increases the noise generated by the bearing assembly.
The high temperatures encountered within an x-ray tube also reduce the scanning performance or throughput of the tube, which is a function of the maximum operating temperature, and specifically the bearing temperature, of the tube. As discussed above, the maximum operating temperature of an x-ray tube is a function of the power and length of x-ray exposure, as well as the time between x-ray exposures. Typically, an x-ray tube is designed to operate at a certain maximum temperature, corresponding to a certain heat capacity and a certain heat dissipation capability for the components within the tube. These limits are generally established with current x-ray routines in mind. However, new routines are continually being developed, routines that may push the limits of existing x-ray tube capabilities. Techniques utilizing higher instantaneous power, longer x-ray exposures, and increased patient throughput are in demand to provide better images and greater patient care. Thus, there is a need to remove as much heat as possible from existing x-ray tubes, as quickly as possible, in order to increase x-ray exposure power and duration before reaching tube operational limits.
The prior art has primarily relied upon removing thermal energy from the x-ray tube through the cooling fluid circulating around the vacuum vessel. It has also relied upon blocking heat to the bearing assembly with high thermal resistance attachments to the target or by placing low emissivity thermal radiation shields between the bearing assembly and the inner diameter of the target. These approaches have been marginally effective, however, they are limited. The cooling fluid methods, for example, are not adequate when the anode end of the x-ray tube cannot be sufficiently exposed to the circulating fluid. Likewise, the shielding methods are generally not adequate as thermal radiation shields have a tendency to heat up, radiating heat to the rotor assembly of the x-ray tube. Thus, the target attachments must be even thinner to prevent heat from being conducted to the bearings. These thin attachments may cause rotor-dynamic problems. Further, placing a thermal radiation shield in the inner bore of the target may also reflect heat back to the target, limiting the performance of the x-ray tube. The shielding methods, in general, do nothing to actually remove heat from an x-ray tube.
The present invention overcomes the problems discussed above and permits greater x-ray tube throughput by providing cooler running bearings and a cooler target at a given tube power. The present invention also reduces thermal growth of the anode, increasing the life and efficiency of the x-ray tube and improving image quality.
In one embodiment, a thermal energy transfer device for use within an x-ray generating device having an anode rotatably supported by a bearing assembly, the x-ray device generating x-rays and residual energy in the form of heat, includes a heat receptor, positioned between the anode and the bearing assembly, for absorbing an amount of the residual energy; and a heat exchanger, in thermal communication with the heat receptor and having an inlet end and an exit end, for carrying a cooling medium and convecting the residual energy absorbed by the heat receptor away from the heat receptor.
In another embodiment, an x-ray generating device includes an anode assembly including a target and a shaft; a bearing structure rotatably supporting the shaft; a cathode assembly disposed at a distance from the anode assembly, the cathode assembly configured to emit electrons that strike the target of the anode assembly and produce x-rays and residual energy in the form of heat; a heat receptor positioned between the anode assembly and the bearing structure, the heat receptor for absorbing an amount of the residual energy; and a heat exchanger, in thermal communication with the heat receptor and having an inlet end and an exit end, the heat exchanger for carrying a cooling medium and conducting an amount of the residual energy absorbed by the heat receptor away from the heat receptor.
In a further embodiment, an x-ray system includes a vacuum vessel having an inner surface forming a vacuum chamber; an anode assembly disposed with the vacuum chamber, the anode assembly including a target; a cathode assembly disposed within the vacuum chamber at a distance from the anode assembly, the cathode assembly configured to emit electrons that strike the target of the anode assembly and produce x-rays and residual energy, said x-rays directed along a focal alignment path;
a rotatable shaft coupled to the vacuum vessel; a bearing assembly comprising a lubricating medium disposed within the vacuum chamber, the bearing assembly providing for rotational movement of the shaft; an annular heat receptor made of a thermally conductive material positioned between the anode assembly and the bearing assembly, the heat receptor having an inner surface with an inner diameter and an outer surface with an outer diameter, the heat receptor for absorbing an amount of the residual energy; and an annular heat exchanger in thermal communication with the heat receptor, the heat exchanger having an inlet and an exit for carrying a cooling medium, the heat exchanger for conducting an amount of the residual energy absorbed by the heat receptor away from the heat receptor.